Charge pump voltage generators are largely used in many integrated circuits for generating a regulated voltage often of opposite sign to the supply voltage and that is maintained substantially constant as the current absorbed by the load circuit varies. An example of a common charge pump voltage generator is shown in FIG. 1. The circuit of FIG. 1 has operating phases in which the pump capacitor CP is charged at a certain supply voltage VDD, alternated with operating phases in which the pump capacitor CP is coupled in anti-parallel to the charge tank capacitor CT, that supplies the electronic circuit with a voltage VNEG of opposite sign in respect to the charge voltage VDD. As long as the voltage VNEG is smaller than the voltage VREF, the pump capacitor CP remains coupled to the supply voltage VDD. When the voltage VNEG exceeds the reference voltage VREF, the capacitor CP charges the tank capacitor CT when the clock signal CK assumes a logic active value, and is charged again at the supply VDD voltage when the clock signal CK becomes a logic null value.
In practice, this loop controls the duty cycle at a constant frequency when the charge current is above a certain threshold that depends on the supply voltage, on the on-resistances RON of the switches SW1 and SW2, on the pump capacitance CP and on the delay of the feedback line formed by the comparator and by the logic gates. The start-up phase of this charge pump generator is critical because the power supply must deliver currents of large peak value. Indeed, at the start-up, the capacitor CP is completely discharged and the current that flows in the supply lines has a peak value IPEAK given by the following equation:
      I    PEAK    =            V      DD              2      ·              R        ON            The on-resistance RON of the switches is generally smaller than 1Ω, thus the current peak IPEAK may typically be larger than above 2A.
When the generator starts up, the tank capacitor CT may also be almost fully discharged, thus the pump capacitor CP will be discharged almost completely during initial charge transfer phases. As a consequence, when a charge phase starts, large current peaks are absorbed from the supply. Only when steady-state conditions are reached, that is when the voltage VNEG approximately equals the voltage VREF, these current peaks will become of normal or acceptable magnitude.